Proton-Based Stereotactic Ablative Body Radiotherapy for Prostate Cancer

Inclusion Criteria: * Male, age ≥ 18 years. * Histological confirmation of adenocarcinoma of prostatic origin. * AJCC 7th edition clinical T1b-fdiabetes mellitus T2b. * AJCC clinical staging rules do not include findings from biopsy or imaging. * Gleason score ≤7 (International Society of Urological Pathologists grades 1-3). * Serum prostate-specific antigen ≤20 ng/mL. * Prostate-specific antigen \<10 ng/mL, if dutasteride within last 90 days or finasteride within last 30 days. * Zubrod performance score 0-1. * Prostate volume \<75-cc, determined by ultrasound, computed tomography or magnetic resonance imaging (modality with lesser volume is acceptable). * If neoadjuvant-concurrent androgen suppression is used, the prostate volume after the start of androgen suppression may be used. * American Urological Association voiding symptom index ≤15. * Ability to complete questionnaire(s) by themselves or with assistance. * Participation in a patient-reported outcomes survey program, inclusive of the Expanded Prostate Index Composite and CTCAE instruments. * Ability to provide informed written consent. Exclusion Criteria: * Radiological- or pathologically-confirmed seminal vesicle invasion, lymph node involvement, or distant metastatic disease. * Radiological- or biopsy-confirmed extraprostatic tumor extension without AJCC 7th edition clinical T3a classification remain eligible. * Prior or anticipated external radiotherapy or brachytherapy, transurethral resection of the prostate, prostatic cryoablation/focused ultrasound/laser therapy, prostatectomy, or prostatic enucleation. * Prior bilateral orchiectomy, planned long-term (\>6 months duration) androgen suppression or peripheral androgen blockade, or chemotherapy or immunotherapy for prostate cancer. * Prior hemi- or total hip arthroplasty. * Diabetes mellitus associated with vascular ulcers or wound healing problems, ulcerative colitis, connective tissue disorder, or chronic (≥120 days) warfarin or clotting factor inhibitor (e.g., rivaroxaban) use planned. * Co-morbid severe concurrent disease which, in the judgment of the investigator, would result in life expectancy \<5 years. * Patients with a left cardiac ventricular assist device are ineligible. * Patients with an implanted medical electronic device (e.g., cardiac pacemaker) remain eligible if device monitoring complies with standard of practice (e.g., Cardiology and Medical Physics evaluation, function checks and during-treatment monitoring). * Immunocompromised due to HIV positive state. * Medical or psychiatric conditions that preclude informed decision-making or adherence. * Prior registration to an Institutional Review Board-approved therapeutic research study that includes proton beam therapy or photon-based radiotherapy (e.g., randomization to proton beam therapy vs. photon-based radiotherapy). * Registration to a non-therapeutic (e.g., biomarker, medical physics) only research study is not an exclusion. * Inability to start radiation per protocol.